The subject of the present invention is a disengageable four-wheel-drive transmission system for a motor vehicle with a transverse gearbox, including a toothed input member meshed with a toothed output member of the gearbox, a front differential which drives the drive shafts of the front wheels of the vehicle, a rear differential which drives the drive shafts of the rear wheels of the vehicle, and means for distributing the torque between the front and rear differentials.
In four-wheel-drive transmission systems of the type defined above, the means for distributing the torque are usually constituted by an epicyclic differential, possibly in association with a FERGUSON-type sliding viscous coupling, a TORSEN-GLEASON distributor, or the like. These known transmission systems do not enable the four-wheel drive to be disengaged selectively when it is not necessary or desired, so as to limit the transmission of torque to the front wheels of the vehicle only or to the rear wheels only. Moreover, the torque is distributed between the front and rear differentials in proportions which are fixed or which can be varied only in proportion to any variations in the road-holding of the front and rear wheels.